This invention relates to a process described in copending applications Ser. No. 593,818, filed July 7, 1975 which process is for purifying gases produced by a gasification of solid or liquid fossil fuels by a treatment with water vapor and oxygen under superatmospheric pressure, which process serves to remove catalyst-deteriorating impurities, such as mono- or polyunsaturated hydrocarbons, mercaptans, HCN, HCl, H.sub.2 S, CS.sub.2, COS, and NH.sub.3, and to desulfurize the gas, so as to produce an exhaust gas which is rich in H.sub.2 S, and comprises cooling and scrubbing the gas under superatmospheric pressure and at normal temperature.
The process of the copending application is characterized in that the raw gas, at a temperature of 150.degree.-170.degree. C, is indirectly cooled to ambient temperature, the condensible hydrocarbons are separated and removed, the gas is subsequently scrubbed with water to remove ammonia in a first scrubbing stage, in which the rate of water addition is controlled to be just sufficient to remove the ammonia, the gas is then scrubbed with a highboiling organic solvent, which is miscible with water and to which sulfur is added, in a second scrubbing stage, in which the rate of solvent addition is controlled in dependence on the solubility of the methylmercaptan, which is to be removed, in the solvent, the gas is subsequently scrubbed with the same solvent in a third scrubbing stage, in which the water content of the solvent is kept at 5-30 mole percent H.sub.2 O and in which H.sub.2 S and COS ar entirely and selectively removed from the gas, and the scrubbing agents from the second and third scrubbing stages are separately regenerated and recycled to the second and third scrubbing stages for re-use.
According to additional features, sulfur is added to the scrubbing agent in the second scrubbing stage in an amount of 0.1 to 10 g, preferably 3 to 8 g, per liter of scrubbing liquid.
The sulfur may be formed in that the hydrogen sulfide contained in the gas which leaves the first scrubbing stage is reacted with the oxygen which is present in the gas so that the oxygen is simultaneously removed from the gas, and in that a soluble activator, preferably a compound which has a boiling point which is approximately as high as, or higher than the high-boiling organic solvent, is added to the scrubbing agent of the third scrubbing stage. A basic nitrogen-containing organic solvent may be used as an activator, e.g., an imidazole derivative, such as 1,2-dimethylimidazole in quantities of 1-10% by weight, preferably 5-15% by weight.
Pyrrolidone or a derivative of pyrrolidone, preferably N-methylpyrrolidone, or polyglycolether, Sulfolan, butyrolactone, morpholine and/or N-methyl- .epsilon.-caprolactam, has been used as a high-boiling organic solvent within the scope of the invention.
The laden scrubbing agents in the second and third scrubbing stages are regenerated by being flashed, possibly with a supply of heat, and by being subsequently stripped with water vapor or inert gas or inert gas and/or being subjected to a vacuum as far as the laden scrubbing agent of the third scrubbing stage is concerned. The regenerated scrubbing agents are recycled in a cycle.
The advantages afforded by the invention of the copending application reside mainly in that the gases produced by a gasification of solid or liquid fuels cannot only be purified but can be selectively desulfurized at the same time.
The gas if first scrubbed with water to remove ammonia. In the second scrubbing stage, an organic solvent is used to remove all catalyst poisons except for H.sub.2 S and COS. These two constituents are finally removed in a third stage. The scrubbing agent requirement is low.
The resulting pure gas may be used for chemical syntheses, e.g., for the production of ammonia, methane, methanol, oxo products or Fischer-Tropsch products. The exhaust gas from the third stage is so highly enriched with hydrogen sulfide that it can be processed by the Claus process to produce elementary sulfur.